The technology used to map to Mars is advancing at a thrilling rate and uncovering all kinds of curiosities. The latest head-scratching image comes from the Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE). It reveals a series of mysterious sandy ridges. No one knows exactly how they got there.

Researchers at the University of Arizona, where HiRISE is based, thinks they're sandy features called traverse aeolian ridges (TARs) that often form in craters or channels. Like sand dunes, they can grow to be dozens of feet high, but scientists don't really know how. "The physical process that produces these features is still mysterious," reads the HiRISE blog. "Most TARs display no evidence of internal structure, so it is difficult to discern exactly how they were formed."

The story of how HiRISE revealed these ridges is just as interesting as the mystery itself. A University of Arizona researcher spotted the TARs while analyzing a digital terrain model of the area which is in the tropics of Mars. These models are produced in stereo, made out of two images of the same spot on the ground but taken from two different angles. This method produces tremendous resolution—less than one foot per pixel—and more topographical data than a typical satellite images. However, triangulating the image requires sophisticated software and lots of manpower.

So mapping Mars like this is going slowly. But it's going. Mysteries like the tropical ridges keep things intriguing. Plus the images themselves are beautiful. [Universe Today via HiRISE]